The present invention relates to a method for preparing a heat sensitive element by spray coating.
Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas, which accept ink, form the printing image areas and the ink-rejecting areas form the background areas.
In the art of photolithography, a photographic material is made image-wise receptive to oily or greasy ink in the photo-exposed (negative working) or in the non-exposed areas (positive working) on a hydrophilic background.
In the production of common lithographic plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used.
Upon image-wise exposure of the light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
On the other hand, methods are known for making printing plates involving the use of imaging elements that are heat sensitive rather than photosensitive. A particular disadvantage of photosensitive imaging elements such as described above for making a printing plate is that they have to be shielded from the light. Furthermore they have a problem of sensitivity in view of the storage stability and they show a lower resolution. The trend towards heat sensitive printing plate precursors is clearly seen on the market.
For example, Research Disclosure no. 33303 of January 1992 discloses a heat sensitive imaging element comprising on a support a cross-linked hydrophilic layer containing thermoplastic polymer particles and an infrared absorbing pigment such as e.g. carbon black. By image-wise exposure to an infrared laser, the thermoplastic polymer particles are image-wise coagulated thereby rendering the surface of the imaging element at these areas ink acceptant without any further development. A disadvantage of this method is that the printing plate obtained is easily damaged since the non-printing areas may become ink accepting when some pressure is applied thereto. Moreover, under critical conditions, the lithographic performance of such a printing plate may be poor and accordingly such printing plate has little lithographic printing latitude.
EP-A-514 145 discloses a heat sensitive imaging element including a coating comprising core-shell particles having a water insoluble heat softenable core component and a shell component which is soluble or swellable in aqueous alkaline medium. Red or infrared laser light directed image-wise at said imaging element causes selected particles to coalesce, at least partially, to form an image and the non-coalesced particles are then selectively removed by means of an aqueous alkaline developer. Afterwards a baking step is performed. However the printing endurance of a so obtained printing plate is low.
EP-A-599 510 discloses a heat sensitive imaging element which comprises a substrate coated with (i) a layer which comprises (1) a disperse phase comprising a water-insoluble heat softenable component A and (2) a binder or continuous phase consisting of a component B which is soluble or swellable in aqueous, preferably aqueous alkaline medium, at least one of components A and B including a reactive group or precursor therefor, such that insolubilization of the layer occurs at elevated temperature and/or on exposure to actinic radiation, and (ii) a substance capable of strongly absorbing radiation and transferring the energy thus obtained as heat to the disperse phase so that at least partial coalescence of the coating occurs. After image-wise irradiation of the imaging element and developing the image-wise irradiated plate, said plate is heated and/or subjected to actinic irradiation to effect insolubilization. However the printing endurance of a so obtained printing plate is low.
EP-A-625 728 discloses an imaging element comprising a layer which is sensitive to UV- and IR-irradiation and which can be positive or negative working. This layer comprises a resole resin, a novolac resin, a latent Bronsted acid and an IR-absorbing substance. The printing results of a lithographic plate obtained by irradiating and developing said imaging element are poor.
U.S. Pat. No. 5,340,699 is almost identical with EP-A-625 728 but discloses the method for obtaining a negative working IR-laser recording imaging element. The IR-sensitive layer comprises a resole resin, a novolac resin, a latent Bronsted acid and an IR-absorbing substance. The printing results of a lithographic plate obtained by irradiating and developing said imaging element are poor.
U.S. Pat. No. 4,708,925 discloses a positive working imaging element including a photosensitive composition comprising an alkali-soluble novolac resin and an onium-salt. This composition can optionally contain an IR-sensitizer. After image-wise exposing said imaging element to UVxe2x80x94visiblexe2x80x94or eventually IR-radiation followed by a development step with an aqueous alkali liquid there is obtained a positive working printing plate. The printing results of a lithographic plate obtained by irradiating and developing said imaging element are poor.
EP-A-96 200 972.6 discloses a heat sensitive imaging element comprising on a hydrophilic surface of a lithographic base an image forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a water insoluble alkali soluble or swellable resin and a compound capable of converting light into heat, said compound being present in said image forming layer or a layer adjacent thereto, wherein said alkali swellable or soluble resin comprises phenolic hydroxy groups and/or carboxyl groups. However by exposure with short pixel times of said heat-sensitive imaging element there occurs ablation on the exposed areas resulting in an insufficient ink acceptance.
Analogous imaging elements comprising on a hydrophilic surface of a lithographic base an image forming layer comprising hydrophobic thermoplastic polymer particles dispersed in a water or alkali soluble or swellable resin and a compound capable of converting light into heat, said compound being present in said image forming layer or a layer adjacent thereto are disclosed in e.g. EP-A-770 494, EP-A-770 495, EP-A-770 496, EP-A-770 497, EP-A-773 112, EP-A-773 113, EP-A-774 364, EP-A-800 928, EP-A-96 202 685, EP-A-96 203 003, EP-A-96 203 004 and EP-A-96 203 633. In most of these applications poly(meth)acrylate latices are used as thermoplastic polymer particles and no specific hydrophilic resin is mentioned In most cases carbon black or an IR-dye are mentioned as the compound capable of converting light into heat.
In order to prepare an imaging element as described above, that is processable on the press, preferably IR-dyes should be used. Carbon black causes indeed a soiling on the press when removing the unexposed areas. On the other hand when using IR-dyes the unexposed areas are not completely dissolved when developing on the press resulting in scumming.
The appliance of the coatings which are used at the preparation of lithographic precursor plates happens mostly with coating techniques such as dipcoating, cascade coating and curtain coating. The use of spray techniques for applying lithographic layers fails usually at the attainable level of cosmetic quality of the end product. The conditions for high qualitative lithographic materials (thermal printing plates well or not processable on press) whereat high resolution, sensitivity and reproducing characteristics are required, are very high with relation with the cosmetic quality of said printing plate. This cosmetic quality can be translated as the presence of lines, the general evenness and the presence of a mottle pattern. This mottle pattern appears at the slightest presence clearly in the printing process of large raster surfaces.
It is an object of the present invention to provide the necessary parameters for obtaining a spray-coated layer with excellent cosmetic quality.
According to the present invention there is provided a method for obtaining a high quality printing plate by spraying a spray solution on a receiving surface, which is not a grained and anodized aluminum surface, characterized in that the pressure factor (PF) is lower than 200 mN/m, wherein
PF=P/dxc3x97("sgr"+xcex8 mN/mxc2x0)
PF: Pressure Factor (mN/m)
P: Spray Profile (mm)
d: distance between spray head and receiving surface (mm)
"sgr": surface tension (mN/m).
xcex8: Dynamic contact angle of the receiving surface with water at 2 s contact time
To define the spray profile, under well-defined settings from solution and hardware, during 1 pass of the rotating drum, a line is sprayed without transverse movement of the spray head. To obtain the right spray pattern, as substrate a well swelling receiving layer, comprising gelatin, polyvinylpyrrolidone and polyethylene glycol (Agfajet Photograde Paper HP Glossy 165(trademark), commercially available from Agfa-Gevaert) was used. This results in an immediate freezing of the spray pattern without the possibility of transverse flowing of the spray solution over the receiving surface. From this line, with the use of microdensitometry, the density profile of the line is measured. In the next step, the width at half height of this profile is divided by the total height (the maximum density) of the profile. This value is referred as profile value (P).
The spray profile is determined by the air pressure of the spraying head, by the flow rate of the spraying head and by the nature of the receiving surface.
This value lies preferably between 50 and 220 mm although this value has to be considered in the context of the given equation. The surface tension of the spray solution lies preferably between 22 mN/m and 60 mN/m.
The pressure factor is preferably lower than 125 mN/m
The distance between the spray head and the receiving member lies preferably between 25 and 100 mm.
The spray solution is preferably an aqueous solution, which may comprise surfactants, preferably fluorosurfactants. The viscosity of the spraying solution is preferably at least 1.5 mPa.s.
The dynamic contact angle of the receiving surface with water is preferably lower than 600 after 2 s contact time.
The receiving surface can be a drum with a lithographic surface, which can be incorporated in a printing machine. The receiving surface can be a lithographic surface mounted on a drum.
A preferred spraying solution is a dispersion of hydrophobic thermoplastic polymer particles in a hydrophilic binder. Said solution preferably includes thermoplastic particles of a homopolymer or a copolymer of styrene and a hydrophilic polymer containing carboxyl groups, and further a compound capable of converting light into heat.
Such solutions, suitable for spraying heat sensitive imaging elements are described with their exposure and development in EP-A-98 200 187.
The receiving element is a lithographic surface on a support layer with a hydrophilic support that is not a grained and anodized aluminum support. Preferably this lithographic surface on a support layer comprises a flexible support coated with a hydrophilic, more preferably with a hardened hydrophilic layer.
The imaging element, obtained by spraying the spray solution on the receiving element can after exposure to an IR-laser be developed by rinsing the element with an aqueous solution. Preferably the exposed imaging element is mounted directly on the press.